Electrolytic recovery of metal from solutions and slurries is known. It has enjoyed considerable success in the electrowinning or refining of metals from clear solutions and moderate success in the direct electrowinning of metals from slurries of ores and concentrates.
A cell that has proven especially useful in the electrowinning of metals, particularly copper and silver, directly from slurries of ore or concentrates thereof is one of recent developement in which a cathode is reciprocated between spaced anodes, the slurry is highly agitated and current densities of as high as 75-100 amps/ft.sup.2 of cathode area are employed to recover plates of high grade metal at high efficiencies.
Although the foregoing cell has been successful in electrowinning of metal, it suffers from certain drawbacks that adversely affect its efficiency and cost. For instance, stopping and starting of the cathode during oscillation frequently causes premature shedding of product plates from the cathode. This necessitates frequent shutdown for plate removal; and is a particular disadvantage in cells with large cathodes. Also, in such a cell, it is not possible to control the relative quantities of slurry flowed against the anode and cathode respectively which means that there must be sufficient electrode (particularly anode) area to satisfy all system needs. If reaction conditions promote low efficiency then it is necessary to provide more anode area to compensate.
As will be more particularly pointed out hereinafter, we have found that by exercising control of the relative volume of slurry flowed against the electrodes, we can maximize their efficiency thus avoiding the need for excess electrode area. Illustratively, electrowinning with the use of an electrolyte having a high ferric iron content and at high current densities has been demonstrated as a viable process. However, such process, when employed in prior art cells, suffers from the disadvantage that anode leaching is inefficient thus requiring that the anode area be several times the cathode area. This requires larger cells thus increasing initial cost and expense of operation. Moreover, even in the above-mentioned reciprocating cathode cell, high current densities (above 100 amp/ft.sup.2) often result in an excessive buildup of dendritic copper adjacent the end and bottom edges of the cathode as well as polarization which causes burned or powdery plates, both of which detract from plate quality.